Our long-term objective of this proposal is to gain an understanding at the molecular level of how the tumor microenvironment determines tumor growth and progression. Decades of cancer research has come to a conclusion that cancer is essentially a genetic disease acquiring dynamic changes in the genome. The development of cancer in humans requires a complex succession of genetic alterations over time, conferring selective growth advantage on cells undergoing progressive transformation. These genetic changes result in activation of oncogenes and inactivation of tumor-suppressor genes for tumor development. Various DNA repair mechanisms safeguard the genomic integrity in normal cells by correcting mutations arising from myriad types of damage. Germline mutations of DNA repair genes have been linked to diverse types of hereditary cancers. However, no such mutations are generally found to be responsible for the development of sporadic cancers. Apart from tumor itself, the tumor microenvironment, hypoxia in particular, has been associated with increased genetic instability in cancers, presumably attributable to tumor progression and resistance to chemotherapy and radiotherapy, even though the underlying mechanisms remain obscure. We have recently shown that the hypoxia-inducible factor 1a (HIF-1a), a central transcription factor of the hypoxic response, induces genetic instability by inhibiting DNA repair gene expression via a novel HIF-1a-c-Myc pathway, suggesting the involvement of functional impairment of DNA repair in tumor development and progression. Despite the heavy implication in tumor growth and angiogenesis, HIF-2a, an isoform of HIF-1a, fails to inhibit DNA repair because of threonine phosphorylation. In this proposal, we study the role of HIF-1a and HIF-2a in tumor development and progression. Specific Aim 1 and Specific Aim 2 test the hypothesis that the induction of genetic instability through activation of HIF-1a-Myc pathway confers malignant traits such as invasion on cultured tumor cells in vitro and tumorigenicity, local invasion, and metastasis in tumor xenograft models. Specific Aim 3 investigates the requirement of phosphorylation for HIF-2a activities and in turn tumor growth in both cell culture and mouse models We believe that this proposal addresses the fundamental mechanisms underlying the dynamic nature of tumor growth and progression and will build a molecular basis for the development of novel therapeutics. PUBLIC HEALTH RELEVANCE Cancer is, in essence, a genetic disease acquiring dynamic changes in the genome. This proposal tackles the obscure mechanisms underlying genetic instability of cancer cells, an insurmountable problem in the fight against cancer. We anticipate this study will help define the molecular basis for the development of novel therapeutics.